The present invention relates to a lift cylinder, and particularly to an improvement in a lift cylinder which is a single action hydraulic cylinder used for a fork lift.
For example, as a lift cylinder used for a fork lift, the present inventor has developed a single action hydraulic cylinder, for example, as shown in FIG. 3. In this proposal, when pressure oil from an external hydraulic source P not shown in the figure but indicated merely by a symbol is supplied to a pressure chamber R which is a piston side chamber defined within a cylinder body 1 by a piston 2 slidably provided in the cylinder body 1, the pressure chamber R is enlarged for extending operation.
The above lift cylinder is operated to be contracted when a rod body 3 having a proximal end connected to the piston 2 is moved into the cylinder body 1 by the own weight in the rod body 3, or the rod body 3 is moved into the cylinder body 1 by a load acting on the rod body 3, whereby oil in the pressure chamber R is discharged to the hydraulic source P so that the pressure chamber R is contracted.
On the other hand, in the above lift cylinder, a non-piston side chamber defined by the piston 2 in the cylinder body 1 is made to be an air chamber A serving as a closed space as shown, and therefore, at the time of extending operation in which the rod body 3 is slipped out of the cylinder body 1, the air chamber A is compressed to exhibit a spring force. That is, this provides a function that at the time of contraction operation in which the rod body 3 is moved into the cylinder body 1, the contraction operation is assisted.
When oil leaked out of a seal S to the pressure chamber R is gradually stayed in the air chamber A, the volume of the air chamber A is reduced, and accordingly, in a case where the air spring force caused by the air chamber A in the lift cylinder becomes excessively high, oil moved into the air chamber A due to the opening operation of a check valve 4 held by the piston 2 is returned to the pressure chamber R to maintain the air spring force caused by the air chamber A in a set value.
Therefore, in this lift cylinder, the provision of the check valve 4 causes the air spring force caused by the air chamber A not to be excessively high, and accordingly, for example, a seal member not shown disposed at an open end of the cylinder body 1 to prevent leakage in the rod body 3 is prevented from being broken, thus improving durability in the lift cylinder.
However, in the above-described lift cylinder, there is the possibility to be pointed out to involve an inconvenience that when the lift cylinder is assembled, many work steps are necessary to fail to expect an improvement in workability at the time of assembly.
That is, in the lift cylinder shown in FIG. 3, the check valve 4 is disposed internally of a fitting part 2a formed to be tubular in the piston 2, but its constituent parts include a valve seat member 41, a poppet valve 42, a bias spring 43, a spring receiver 44 and a washer 45, and in a state that the valve seat member 41 is stopped at an upper end level-difference part 2b of the fitting part 2a, a snap ring 5 is fitted in inside at the lower end of the fitting part 2a to thereby prevent slipping out from the inside of the fitting part 2a which is a predetermined position.
Therefore, it is of course that the check valve 4 is disposed before the piston 2 is connected to the rod body 3, and the position of arrangement thereof is at a deep location, and therefore, the above-described parts, that is, many parts are to be arranged using a jig and through a communication hole 2c bored in a shaft center of the piston 2.
As a result, in the above-described lift cylinder, the assembling work of the check valve 4 is not easy, and accordingly, many work steps for the assembly of the check valve 4 are necessary, failing to expect an improvement in workability at the time of assembling the lift cylinder.
The present invention has been accomplished in view of the foregoing, and has its object to provide a lift cylinder which facilitates assembling work of a check valve and which will be optimum for expecting an improvement in workability at the time of assembly.
For achieving the aforementioned object, the present invention provides a lift cylinder wherein a rod body is slidably received in a cylinder body through a piston, the piston defining a pressure chamber and an air chamber within the cylinder body, the piston being provided internally with a check valve for allowing only a flow of oil from the air chamber to the pressure chamber and a cushion ring for effecting a cushion near the maximum contraction, and when the rod body is operated to be expanded and contracted at the time of supply and discharge of pressure oil to the pressure chamber and the air spring force of the air chamber becomes excessively high due to accumulation of oil which leaks out of a seal in the outer periphery of a piston from the pressure chamber and flows into the air chamber, oil is returned to the pressure chamber through the check valve, characterized in that the check valve comprises a tubular valve seat member inserted in a shaft center part of the piston, a poppet valve received in the valve seat member and seated or unseated on the inner peripheral seat surface of the valve seat member, a spring receiver secured to an open end of the valve seat member, and a bias spring carried on the spring receiver to bias the poppet valve in a direction of closing it, and the cushion ring is arranged opposedly in proximity of the valve seat member.
In this case, alternatively, the piston has a body in sliding contact with the cylinder body, and a tubular fitting part erected from the body and inserted into the piston rod, the check valve is arranged over the shaft center part of the body and the fitting part, and the cushion ring is arranged on the shaft center part of the body movably while opposing to the check valve.
Likewise, the piston has a body in sliding contact with the cylinder body, and a tubular fitting part erected from the body and inserted into the piston rod, the check valve is arranged on the shaft center part of the body, and the cushion ring is arranged on the shaft center part of the body movably while opposing to the check valve.